There were many lengthy discussions on this over the years, some say they've hit the governor, and some claim there is no governor...
http://www.mazdas247.com/forum/showthread.php?123675494-Top-Speed-Run...
Here is my take in addition to above posts:
- Could be lots of carbon build up. If you rev up the engine to redline momentarily, does a lot of black smoke come out? [Excess carbon build up will create more compression, and would cause pre-detonation]
- Are you using good quality gas...